1. Field of the Invention
The present invention relates to cooling of a multi-chip module. More precisely, the present invention relates to embedding heat pipes into the substrate of a multi-chip module to cool semiconductor chips mounted on the module.
2. Art Background
An important objective of computer design is to fit the greatest number of semiconductor chips or ICs into the smallest space. Factors such as substrate design, interconnect design, cooling method, density of chip placement, etc., have great bearing on the ultimate performance of the computer. The tendency of designers to minimize the size of the computer while maximizing its computing power has led to more and more densely packed IC chips. The density of interconnects that provide the signal path between ICs must concurrently rise. Unfortunately, these densely networked interconnects have a propensity to generate heat.
By the same token, higher computing power translates to a faster rate at which instructions are executed. To execute more instructions per second, the circuits must operate at a higher frequency. Operating at a higher frequency requires higher energy input and consequently more energy is generated in the devices. A byproduct of high energy input is heat.
Higher computing power also means the ability to execute larger and larger sets of instructions. As a result, the semiconductor devices used within a given area must have greater memory capacity to accommodate the increase. Thus, more energy is required to operate the increased number of memory devices. Again, more energy input results in more energy appearing as heat. It follows then that cooling of these devices should be a major concern.
In prior art computers, the circuits were simply cooled by air convection circulated by a fan. But when the fan was used in conjunction with high density, multi-chip, main frame computers, the large volume of air needed for cooling necessitated powerful blowers and large ducts. Such clumsy structures in the computer occupied precious space and were noisy too.
There have been other approaches to cooling ICs. For example, U.S. Pat. No. 4,748,495 to Kucharek discloses a package for cooling a high density array of IC chips and their interconnections. In this arrangement, the IC chips are mounted in a generally planar array with individual heat sinks connected to the ICs separated by flexible membrane mounts. All of the cooling structure are thus mounted on top of the ICs, separated by the membrane mounts. Cooling fluid is then pumped through the cooling structures, thus carrying coolant past the areas above the ICs.
Likewise, U.S. Pat. No. 4,879,629 to Tustaniwskyj et al. discloses a liquid cooled multi-chip integrated circuit module that incorporates a seamless compliant member for leak proof operation. In particular, heat sinks are disposed immediately on top of the ICs while on top of the heat sinks are disposed channels that carry liquid coolant wherein the channel is incorporated into a rigid cover. A compliant member seals off the channel area from the chip area to eliminate the possibility of leakage of liquid coolant.
Because of variations in the way ICs are mounted to the substrate, the top surface of the IC may be tilted at different angles which impairs heat conduction to the heat sink. U.S. Pat. No. 5,005,638 to Goth et al. provides structure to ensure solid contact between the heat sink and the IC. Goth discloses barrel shaped pistons that are spring loaded and biased toward the IC chips such that any minor tilt in the chips are compensated by the springs. Heat then rises from the IC chips up through the barrel shaped pistons and into a large body heat sink. Coolant is then pumped through the heat sink to assist in heat dissipation.
Unfortunately, with the coolant fluid disposed above the chips as in the prior art, there is always a possibility of coolant leakage. If such leakage should take place, assuming the coolant is electrically conductive, the malfunction would be catastrophic. Even if the coolant were not conductive, it would contaminate the chips leading to other reliability problems. Furthermore, the structures needed for conduction of fluid and contact between the heat sink and the chips are typically very complex. These intricate structures require a great deal of attention during assembly and are usually expensive to fabricate.
Finally, U.S. Pat. No. 5,095,404 to Chao discloses a cooling apparatus mounted to a printed circuit board. The printed circuit board has a hole for receiving the cooling apparatus comprising several layers including a heat spreader, a heat pipe, and heat sinks. The heat spreader engages a semiconductor chip through the hole in the printed circuit board. The heat spreader is mounted to the under side of the printed circuit board, and the heat pipe is mounted to the heat spreader through intervening layers. Heat sinks are coupled to the heat pipe. Unfortunately, such a multi-layer cooling apparatus does not conduct heat efficiently since each layer adds a resistive barrier to heat conduction. Moreover, such an externally mounted cooling apparatus increases the space required by the printed circuit board, thereby increasing the overall form factor of the system containing the printed circuit board.